I've been buying fabric all my life I think! Started sewing my own clothes at thirteen.
My answer is buy it anywhere you see what you like. And let people around you know that you are a quilter and you will most likely be the recipient of their grandmother's scraps if they don't sew.
Yard sales, Craig's list and thrift shops can also be good sources. The For Sale or Trade section of this board is always a great place to shop.
Any time you see a shop that might be a fabric store, check it out!
And if you like doing totes, check out upholstery and drapery manufacturers and custom shops for their scraps or outdated sample books.
Happy quilting and fabric shopping!